In the manufacture of fiber from molten material, such as molten glasses, it has been common practice to use a bushing made of precious metal alloy. Precious metals are platinum, rhodium, palladium, ruthenium, and iridium. The bushings are electrically heated by their own resistance and are box-like, open on the top and comprise an orifice plate containing many nozzles or tips welded therein, sidewalls, end walls, terminals on the endwalls for connecting electrical cables, a top flange for contacting the underneath side of a forehearth, and usually a perforated plate or screen parallel with, but mounted above, the orifice plate. Usually the bushings are made by cutting the parts from alloy of desired thickness and welding the parts together with similar alloy, but a part or all of the bushing can be made by casting and/or drawing as shown by U.S. Pat. Nos. 4,207,086 and 4,078,413, which disclosures are hereby incorporated by reference.
The depth of the bushing between the top of the bushing and the top of the orifice plate was thought important to allow the glass to homogenize in temperature and viscosity before reaching the nozzles so that there would be uniform flow through all of the nozzles. As the molten glass flows through the openings in the refractory floor of the forehearth, the refractories forming the openings are cooler than the molten glass and remove heat from the molten glass in contact with the refractories. This creates a temperature profile in the column of molten glass entering the bushing. Usually a perforated plate or screen spans the opening close to the top of the bushing and is electrically heated to attempt to remove temperature differences in the glass and to bring the glass to a uniform, desired temperature for fiberization.
Removing the screen, and the sidewalls, from the bushing has been attempted with a much higher fiber break rate just below the bushing being the consistent result. It has been concluded that this higher break rate is caused by increased variation in the temperature and viscosity of the molten glass entering the nozzles due to the absence of the screen or the distance between the screen and the orifice plate, i.e. an insufficient length of the sidewalls in either case. As a result, the sidewalls were increased to try to achieve better glass uniformity. To conserve costly alloy, at least a substantial portion of the bushing sidewalls was made thinner than the orifice plate and the height of the sidewalls and endwalls exceeded at least three quarters of an inch, usually one inches or more, and frequently up to one and one half inch or more as evidenced by U.S. Pat. Nos. 4,272,271; 4,662,922 and 5,244,483, the disclosures of which are hereby incorporated by reference.
Because of the temperatures of the molten material being fiberized and the corrosiveness of the molten material, it is necessary to make the bushing from very expensive alloys like platinum-rhodium alloys typically containing 10-30% rhodium. These alloys are very expensive, usually exceeding seven thousand dollars per pound, and it is not unusual to have many tens of thousands of dollars tied up in each fiberizing position in alloy alone. Thus it has been desirable for decades to reduce the amount of alloy required while at least maintaining, and preferably improving, the variation in the temperature of the molten material entering the nozzles.